Wide area wireless networks are often arranged to allow wireless communication devices (WCDs) to use an extension of the Internet Protocol that supports WCD mobility, such as mobile Internet Protocol (IP) or proxy mobile IP (PMIP). When a WCD employs mobile IP, the WCD is assigned a home IP address by a mobile IP home agent in the WCD's home network. When the WCD roams to a foreign network, the WCD directly or indirectly registers with a mobile IP foreign agent in the foreign network and informs the home agent that it (the WCD) is being granted network access via the foreign agent. Then, communications between the WCD and correspondent nodes are encapsulated (tunneled) between the foreign agent and the home agent.
In the case of PMIP, the WCD may register with a PMIP gateway, which is typically in the foreign network. The PMIP gateway conducts a mobile IP transaction with the mobile IP home agent on behalf of the WCD, such that the WCD is assigned a home IP address by a mobile IP home agent. In this way, the WCD may not be aware of the mobile IP transactions between the PMIP gateway and the home agent, and may not even need to support the mobile IP protocol. Similar to mobile IP, communications between the WCD and correspondent nodes are tunneled between the PMIP gateway and the home agent.
These mobile IP and PMIP tunnels serve at least two purposes. First, since the home agent assigns the WCD a home IP address from the home network, any communications from correspondent nodes to this home IP address will be initially routed to the home network, rather than directly to the WCD. Thus, the tunneling delivers these communications from the home network to the WCD. Second, the WCD may have registered to use services that are available in the home network. Since the tunneling routes all of the WCD's communications via the home network, the WCD can still take advantage of these services while roaming.
Nonetheless, the physical and/or topological distance between the foreign agent and the home agent (in the case of mobile IP), or between the PMIP gateway and the home agent (in the case of PMIP) may be significant. Thus, it can be expensive to backhaul communications from thousands, or even millions, of WCDs to and from home agents.